1. Field of the Invention
The present invention relates, in general, to motor and gearbox drives and, more specifically, to motor and gearbox drives with right-angled gearboxes and, even more specifically, to motor and gearbox drives for vehicle windshield wipers.
2. Description of the Art
Electric motors having gear reduction are widely employed for many industrial applications. Typically, the gear reduction is provided by a gear train formed of at least two meshing gear members, one of which is driven by the motor output shaft and may be actually incorporated into the structure of the motor output shaft, and the second fixed on an axle or shaft thereby rotating the shaft upon rotation of the motor output shaft.
In a typical application of such a motor/gearbox drive, windshield arms and blades are attached to a conventional linkage which includes a crank arm fixedly mounted on the rotatable output shaft of a motor/gear drive. Within the motor/gearbox housing, a worm gear is formed on the motor output shaft. One end of the shaft is rotatably mounted in a bore formed in the housing. An intermediate end of the shaft is also supported in a bearing mounted in the housing.
Cost reduction in the motor/gearbox drive has focused on the worm gear shaft and related components. Cost reduction can be achieved in this area by using a smaller diameter worm gear shaft. However, without support at the end of the shaft, the gear train fails at less than the required torque output. Such failure is a result of excessive deflection of the worm gear shaft. To control the deflection of the shaft, a bearing surface is required at the end of the shaft. A thrust bearing surface is also required to control axial movement of the shaft. One solution is to machine a bore in the housing, which then receives a press-in bearing. However, the location of the machined bore cannot be held to the necessary tolerance for proper location of the bearing-to-shaft journal. Proper location of this journal is essential for low noise and high efficiency of the motor/gearbox drive. If the bearing is more than a few thousandths of an inch out of position, excessive noise and friction result.
Axial end play of the gear shaft must also be controlled by minimizing such axial movement in order to prevent noise. It is known to provide a drilled and tapped bore in the housing axially in line with the worm gear shaft, which bore receives a threaded screw carrying a molded elastomer or resilient end cap. The screw is threaded into the tapped bore a sufficient distance to bring the end cap into engagement with the shaft. While this minimizes axial movement of the shaft to a certain extent, such an arrangement introduces other problems, the most significant of which is the drilled bore which forms a new water path entry into the motor/gearbox housing. In addition, the end cap applies force to the shaft and thereby controls the gear meshing.
Another solution to the radial and axial movement problems associated with a smaller diameter worm gear shaft is to provide another bearing in the gearbox housing to support the end of the shaft. However, this introduces an added cost into the motor gearbox drive.
Thus, it would be desirable to provide a motor/gearbox drive having molded sleeve and thrust elements or bearing surfaces which enable a smaller than normal diameter output drive shaft to be employed while still preventing excess radial deflection and excess axial movement of the drive shaft. It would also be desirable to provide such thrust and sleeve elements which minimize noise during operation of the motor/gear drive. It would also be desirable to provide sleeve and thrust bearing surfaces for a motor gear drive which can be easily integrated with existing motor/gearbox production methods for low manufacturing costs.
The present invention is a unique method and apparatus for providing a sleeve and a thrust member in a motor/gearbox housing to support the motor/gear drive shaft against radial and axial loads.
The apparatus includes a motor/gear drive arrangement where a motor drive shaft has a worm gear carried thereon and a tip end terminating in an end wall, and a bore is formed in a gearbox housing coaxial with the output shaft.
In one aspect, the invention comprises an improvement including an annular sleeve concentrically disposed about the tip end portion of the output shaft and nominally spaced from the tip end portion. The tip end portion of the drive shaft, under radial loads acting on the drive shaft, engages the sleeve, with the sleeve limiting further radial movement or deflection of the drive shaft.
In another aspect of the invention, an injection molded thrust member is disposed in the bore in the housing in coaxial registry with the end wall of the shaft. The engagement of the thrust member with the end wall of the output shaft prevents axial movement of the drive shaft.
According to one aspect of the invention, at least one and preferably each of the sleeve and the thrust member are formed by injection molding. More specifically, the sleeve and tubular member are each injection molded within the housing during the assembly of the motor/gear drive.
The sleeve preferably has an inner diameter which is nominally larger than the outer diameter of the tip end portion of the drive shaft such that the tip end portion of the drive shaft nominally rotates within the bore in the sleeve during normal operation of the drive shaft.
Another aspect of the present invention defines a unique method of manufacturing a motor/gear drive wherein the motor has a drive shaft carrying a worm gear, and a tip end portion terminating in an end wall, the method comprising:
forming a bore in a housing surrounding the motor/gear drive, the bore having a first bore portion of a first diameter; and
forming a sleeve having a through bore in the first bore portion of the housing.
The sleeve is preferably formed by the steps:
forming a shoulder between the first bore portion and a second bore portion;
forming a first gate in the housing communicating with the first bore portion; and
inserting a mold core into the housing, the mold core having a first end portion with a diameter larger than the outer diameter of the tip end portion of the drive shaft and a second larger diameter portion having a shoulder formed between the first and second portions sealingly closing the first bore portion, the first bore portion and the tip end portion of the mold core forming an interior cavity;
injecting molten plastic into the interior cavity through the first gate to form the sleeve having an inner diameter surface surrounding a bore; and
removing the mold core.
The method also includes forming a shoulder on the mold core which is engagable with a bearing mounting surface in the housing to concentrically align the mold core with the first and second bore portions in the housing to concentrically align the sleeve with the first bore portion.
In yet another aspect, a first flange is formed on the housing and is engaged with a mating flange formed on the mold core. Engagement of the two flanges aligns the later formed sleeve concentrically about a longitudinal axis of the drive shaft when the drive shaft is engaged with the sleeve.
In yet another aspect of the present invention, the method further comprises the steps of:
forming a second gate in the housing communicating with the second bore portion;
forming an end wall of the drive shaft with an outer diameter larger than the diameter of the second bore portion;
disposing the end wall of the drive shaft to sealingly close off an end of the second bore portion;
inserting the drive shaft of the motor/gear drive into the housing with the tip end portion of the drive shaft extending through the first bore portion;
disposing the end wall of the drive shaft to sealing close the second bore portion; and
injecting molten plastic through the second gate into the second bore portion to form a thrust member in the second bore portion in registry with the end wall of the drive shaft.
The unique thrust member and sleeve of the present invention manufactured by the unique inventive method enables a smaller diameter and therefore a lower cost motor/gear drive shaft carrying a worm gear to be rotatably mounted in a housing and supported against radial and axial movement for reduced noise and friction.
The unique sleeve and thrust member of the present invention also contribute to a reduced manufacturing cost for the motor/gear drive since a costly bearing is not required to support the end portion of the motor/gear drive shaft. Further, the sleeve and thrust member are uniquely formed by an injection molding process utilizing gates formed in the motor/gear drive housing. This reduces assembly steps and part count. In addition, the injection molding of the sleeve and the thrust member can be integrated into an existing motor/gear drive assembly process for a simplified manufacturing of the motor/gear drive.
The use of separate gates directed to separately closed bore portions in the housing used to form the sleeve and thrust member, respectively, enable different materials to be used to form the sleeve and the thrust member thereby enabling the materials to be engineered to meet the different radial and/or axial loads of the sleeve and the thrust member.